1. Field of the Invention
The present invention relates to a soft-focus lens system, and in particular, relates thereto in which a soft-focus photographing mode can be switched to a normal photographing mode without soft-focus effects or vice versa, and which can be used in a single lens reflex (SLR) camera.
2. Description of the Prior Art
A soft-focus lens system of the above type has been disclosed in, e.g., Japanese Unexamined Patent Publication (hereinafter, JUPP) No. 8-86957. In the lens system of this publication, when the normal photographing mode is switched to the soft-focus photographing mode or vice versa, the focal point and the focal length are inevitably varied. If this occurs before and after the switching of the photographing modes, a focusing operation has to be done again. Furthermore, if an attempt is made to automatically correct the varied focal point and focal length by a predetermined mechanism, the structure of the lens system has to become complicated.
Still further, in a lens system which has been disclosed in, e.g., JUPP No. 8-248310, the change of the focal length caused by the switching of the above photographing modes is cancelled by moving another lens system. However, such an arrangement is a type of a zoom mechanism, which also makes the structure of the lens system complicated.
In the above conventional soft-focus lens systems, focusing is performed by advancing the most object-side lens group, or by advancing the entire lens system. As a result, the overall length of the lens system becomes longer. Moreover, the weight of a lens group(s) to perform focusing is heavy, so that it is difficult for a user to easily handle and carry the camera with him/her. Furthermore, it is not suitable to incorporate an automatic focusing (AF) function into such a lens system.
In addition to the above, it has been known that a soft-focus lens system, which (i) can form a clear image in the normal photographing mode, and (ii) can generate uniform flare from the center of the image filed to the periphery thereof in the soft-focus photographing mode, has utilized spherical aberration occurred in lens elements. However, there are requirements that options on the effects of soft-focusing, in addition to the effects of soft-focusing obtained by spherical aberration, be increased in order to widen photographic expressions.
The present invention is to provide a soft-focus lens system which (i) can maintain the focal point and the focal length before and after the switching of the soft-focus photographing mode to a normal photographing mode or vice versa, and which (ii) can dispense with a complicated movement of the lens groups, (iii) can avoid increasing the weight of a lens group(s) to perform focusing, and (iv) can increase options on the effects of soft-focusing in order to widen photographic expressions by utilizing not only spherical aberration but also filed curvature.
As a first aspect of the present invention, there is provided a soft-focus lens system including a positive master lens group (hereinafter, master lens group) and a negative soft-focus lens group (hereinafter soft-focus lens group). The soft-focus lens group includes at least two moveable sub lens groups, i.e., a positive sub lens group and a negative sub lens group, which are independently moveable with respect to the master lens group. The positive sub lens group and the negative sub lens group are moved to set the normal photographing mode or the soft-focus photographing mode in which larger spherical aberration is occurred than in the normal photographing mode. Furthermore, each of the sub lens groups is arranged to move with respect to the master lens group and the other of the sub lens groups so that the focal point and the focal length are not substantially varied before and after the switching of the soft-focus photographing mode to the normal photographing mode or vice versa.
Due to the following reasons, the soft-focus lens group has negative power:
(1) the normal photographing mode is switched to the soft-focus photographing mode or vice versa by changing the distance between the positive sub lens group and the negative sub lens group;
(2) under this arrangement, if the soft-focus lens group has positive power, the changes of the focal point and the focal length are corrected by moving the positive soft-focus lens group toward the image by a predetermined distance;
(3) however, moving the positive soft-focus lens group by a predetermined distance toward the image makes the diameter of a bundle of light rays passing therethrough smaller, thereby spherical aberration becomes smaller;
(4) in other words, such a decrease of spherical aberration is equivalent to canceling the effects of soft-focusing;
(5) on the other hand, if an attempt is made to further increase the positive power of the positive soft-focus lens group so that the change of the focal point due to the effects of soft-focusing is corrected by moving the positive soft-focus lens group by a distance shorter than the above predetermined distance, the positive power of the master lens group becomes weaker; and
(6) as a result, the second principal point moves toward the image, so that the overall length of the lens system becomes longer.
Since there is freedom on the positional relation between the master lens group and the soft-focus lens group, the arrangement thereof in the first aspect of the present invention can be made as follows:
(a)the master lens group and the soft-focus lens group are arranged in this order from the object;
(b) a diaphragm is provided in the master lens group; and
(c) the soft-focus lens group includes the positive sub lens group and the negative sub lens group, in this order from the object, which are constituted by a positive single lens element and a negative single lens element so that an air lens, having a shape of a meniscus lens element with the concave surface facing toward the diaphragm, is formed between the positive single lens element and the negative single lens element.
According to a second aspect of the present invention, a positive lens group to exclusively perform focusing (hereinafter, focusing lens group) to obtain an in-focus state of the object is provided between the master lens group and the soft-focus lens group. In other words, the soft-focus lens system according to the second aspect of the present invention includes the master lens group, the positive focusing lens group (hereinafter, focusing lens group) and the soft-focus lens group, in this order from the object. The arrangement of the soft-focus lens group and the function thereof are the same as the first aspect of the present invention. Each of the sub lens groups of the soft-focus lens group is arranged to move with respect to the master lens group, the focusing lens group and the other of the sub lens groups so that the focal point and the focal length are not substantially varied before and after the switching of the soft-focus photographing mode to the normal photographing mode or vice versa. In the second aspect of the present invention, the master lens group is made constantly immoveable, and the focusing lens group exclusively functions as a lens group to perform focusing to obtain an in-focus state of the object regardless of the positions of the sub lens groups of the soft-focus lens group.
It is practical to provide the diaphragm in the focusing lens group. Similar to the first aspect of the present invention, the positive sub lens group and the negative sub lens group of the soft-focus lens group are constituted by a positive single lens element and a negative single lens element so that an air lens, having a shape of a meniscus lens element with the concave surface facing toward the diaphragm, is formed between these positive and negative lens elements.
Here, a third aspect of the present invention will be discussed. The third aspect of the present invention is devised based on the requirements that the effects of soft-focusing be more beautifully refined. The soft-focus lens system of the third aspect of the present invention includes the master lens group and the soft-focus lens group. The master lens group is constituted by two sub lens groups F1 and F2, and the diaphragm is provided therebetween. Specifically, at least one of the sub lens groups F1 and F2 of the master lens group is made moveable along the optical axis in order to set the normal photographing mode or the soft-focus photographing mode in which larger field curvature is occurred than in the normal photographing mode. On the other hand, similar to the first and second aspects of the present invention, the soft-focus lens group includes at least two moveable sub lens groups, i.e., the positive sub lens group and the negative sub lens group, which are independently moveable with respect to the master lens group and the other of the sub lens groups. The positive sub lens group and the negative sub lens group are moved to set the normal photographing mode or the soft-focusing photographing mode in which larger spherical aberration is occurred than in the normal photographing mode.
In other words, according to the third aspect of the present invention, since the effects of soft-focusing can be attained by utilizing both field curvature and spherical aberration, or either one of field curvature and spherical aberration, the master lens group can therefore be defined as a field-curvature varying lens group, and the soft-focus lens group can be defined as a spherical-aberration varying lens group.
Due to the following reasons, the master lens group is constituted by the diaphragm and the two sub lens groups, at least one of which is made moveable along the optical axis:
(1) the amount of field curvature is determined by the position of the diaphragm, and the positions of the sub lens groups (lens surfaces) before and behind the diaphragm;
(2) usually, the positions of the diaphragm and the sub lens groups are relatively determined so that field curvature can be reduced;
(3) on the other hand, the above means that these positions are relatively determined so that field curvature can be increased, upon necessity; and
(4) according to the third aspect of the present invention, the amount of field curvature can be varied by making at least one of the sub lens group provided before and behind the diaphragm moveable along the optical axis.
In addition to the above, the entire master lens group is required to have positive power, on the other hand, either one of the sub lens groups may have positive or negative power.
The soft-focus lens group according to the first and third aspects of the present invention satisfy the following conditions:
0.05 less than d0/f less than 0.50xe2x80x83xe2x80x83(1)
xe2x88x924.0 less than fB0/f less than xe2x88x920.5xe2x80x83xe2x80x83(2)
wherein
d0 designates the distance between the master lens group and the soft-focus lens group along the optical axis in the normal photographing mode;
fB0 designates the focal length of the soft-focus lens group in the normal photographing mode; and
f designates the focal length of the entire lens system in the normal photographing mode.
The soft-focus lens group according to the second aspect of the present invention satisfies the following conditions:
0.05 less than d0xe2x80x2/fxe2x80x2 less than 0.50xe2x80x83xe2x80x83(1xe2x80x2)
xe2x88x924.0 less than fB0xe2x80x2/fxe2x80x2 less than xe2x88x920.5xe2x80x83xe2x80x83(2xe2x80x2)
wherein
d0xe2x80x2 designates the distance between the focusing lens group and the soft-focus lens group along the optical axis in the normal photographing mode;
fB0xe2x80x2 designates the focal length of the soft-focus lens group in the normal photographing mode; and
fxe2x80x2 designates the focal length of the entire lens system in the normal photographing mode.
The soft-focus lens group according to the second aspect of the present invention satisfies the following conditions:
0.5 less than f3/fxe2x80x2 less than 1.5xe2x80x83xe2x80x83(3)
0.1 less than rR1/fxe2x80x2 less than 2.0xe2x80x83xe2x80x83(4)
wherein
f3 designates the focal length of the focusing lens group; and
rR1 designates the radius of curvature of the most object-side surface of the focusing lens group.
In the third aspect of the present invention, the master lens group (field-curvature varying lens group) is required to have positive power, on the other hand, either one of the sub lens groups F1 and F2 may have positive or negative power. However, it is preferable that the sub lens group F2 behind the diaphragm have positive power, and that the diaphragm move together with the sub lens group F2 when the normal photographing mode is switched to the soft-focus photographing mode in which field curvature largely occurs than in the normal photographing mode, or vice versa. Furthermore, the sub lens group F1 before the diaphragm preferably satisfies the following condition:
|f/fF1| less than 1.0xe2x80x83xe2x80x83(5)
wherein
fF1 designates the focal length of the sub lens group F1 of the master lens group (field-curvature varying lens group).
On the other hand, in order to change the normal photographing mode to the soft-focus photographing mode, or vice versa, the sub lens group F2 can be made immoveable, and only the sub lens group F1 can be made moveable.
The soft-focus lens system according to the third aspect of the present invention satisfies the following conditions:
xe2x88x924.0 less than f/rA less than 1.0xe2x80x83xe2x80x83(6)
0.05 less than dF12/f less than 0.50xe2x80x83xe2x80x83(7)
wherein
rA designates the radius of curvature of the object-side surface of the most-image side lens element in the sub lens group F1 of the master lens group (field-curvature varying lens group); and
dF12 designates the distance between the sub lens groups F1 and F2 in the normal photographing mode.
In the soft-focus lens system of the first and third aspects of the present invention, focusing is preferably performed by advancing the entire lens system, or by advancing the master lens group only, i.e., the front lens-group advancing type. Specifically, in the case of the front lens-group advancing type, the focusing portion and the soft-focus portion can be made completely independent, so that the structure of a mechanism for the lens system can be made simpler. In a lens system of the front lens-group advancing type, since the soft-focus lens group is generally provided behind the diaphragm, such an arrangement is advantageous for the correcting of aberrations.
On the other hand, according to the second aspect of the present invention, since the master lens group is constantly made immoveable, the soft-focus lens system is an inner-focusing type lens system in which the focusing lens group is moveable, between the master lens group and the soft-focus lens group, for performing focusing. Due to this arrangement, the focusing portion and the soft-focus portion can be made completely independent, so that the structure of a mechanism for the lens system can be made simpler. Still further, a lens group to perform focusing, i.e., the focusing lens group, is not heavy, so that such an arrangement is suitable for incorporating an automatic focusing (AF) function into the lens system, and is easy to be handled due to the nature of the inner-focusing type lens system, i.e., the overall length thereof is unchanged.
The present disclosure relates to subject matters contained in Japanese Patent Applications No. 2001-125301 (filed on Apr. 24, 2001), No. 2001-125302 (filed on Apr. 24, 2001) and No. 2001-216409 (filed on Jul. 17, 2001), which are expressly incorporated herein in their entirety.